The invention relates to vehicle seats and particularly to seat support structures of the gas spring type which include a time delay valve for automatically adjusting the seat to a uniform height for any weight occupant as well as a linkage for varying the seat height within a range. Some patents which include these features are: Harder, Jr. et al. U.S. Pat. No. 3,638,897, Swenson et al. U.S. Pat. No. 3,951,373, Costin U.S. Pat. No. 3,954,245 and Rumsey U.S. Pat. No. 4,022,411. In each of these patents the gas spring permits the seat upper to have an extended vertical range of operation which must be of sufficient amplitude to include not only the total range of height adjustment but at least a predetermined amount of dynamic movement above and below the upper and lower limits of the height adjustment range. For example, for a typical seat having a 61/2" overall amplitude of movement in its ride zone, the height adjustment range might be approximately 21/2" so that when the height is adjusted to either the upper or lower extreme of its range, there would still be at least 2" of dynamic movement range available to an occupant seated in the ride zone in both the upper or lower direction. Thus, for the range dimensions just noted, there would be 2" of movement space available above the "mid-ride" position and 41/2" of space below the "mid-ride" position when the seat height is adjusted to the top of its range. Conversely, when the seat is adjusted to the bottom of its range there would be 2" of movement space available below the "mid-ride" position and 4.kappa." of space available above the "mid-ride" position. The excess movement capability in one direction which is necessary to accommodate the height adjustment feature can cause the occupant to have his feet and arms lifted from the foot pedals and steering wheel significantly more than if the ride zone were limited to about 2" on either side of the "mid-ride" position. Furthermore, where the excess movement is in the upper direction, as it would be when the seat is adjusted to the bottom of its height adjustment range, there is the possibility that in a cab with limited ceiling height, a tall person could bump his head on the ceiling. It is known that in a typical suspension seat having a parallelogram or scissors linkage lifting the seat cushion parallel to the floor, a 4" or 100 mm dynamic movement range is quite adequate and should not be substantially exceeded in order to provide effective vibration isolation to a seat occupant while permitting him to retain comfortable control of the floor pedals and steering wheel. Accordingly it would appear desirable to provide a seat support structure which provides the automatic weight adjustment and occupant controlled height adjustment features of prior art seats but with a "protected" ride zone which provides a dynamic movement range which preferably does not exceed about 4" and comprises about 2" above and below the "mid-ride" position of the seat, regardless of movement of the height adjustment. Radke et al. U.S. Pat. No. 3,325,136 assigned to a common assignee, does disclose a "protected" ride zone, but the weight adjustment which centers an occupant of any weight in the ride zone is not automatic in that the occupant must manually squeeze an operating trigger lever to actuate the air valve. The design is somewhat less than completely satisfactory since it not only requires positive acts on the part of the occupant to achieve the weight adjustment, but requires movement of a rotary handle downwardly to go up and upwardly to go down. When the occupant leaves the seat, the seat will naturally rise about 2" to the top of its stroke where it will not only appear to be out of position when unloaded, but will make re-entry to the seat more difficult by reducing the space between the steering wheel and the seat. Where such a suspension is on a passenger seat and no passenger is occupying it, it can rattle somewhat as it vibrates relative to its "up" stop.
Although it has been noted that a dynamic movement range of 4" or less is preferable, such a range could be exceeded without loss of operator control in seats of the type wherein the suspension causes the seat cushion to tilt as it goes up and down in such a manner that the occupant's knee joints or ankle joints remain fixed relative to the floor. Examples of such structures are shown in Radke et al. U.S. Pat. No. 3,356,413 and in Barley Ser. No. 39,636 filed May 16, 1979 and assigned to a common assignee.